Robert
Best bet might be to offer 50% up front, but where is the shop - Chiang Mai or Bangkok?
I'd also try to get a clause written into the contract saying that he would buy the bike back for the full price if he cannot deliver on the rego.
Red Barron in Bkk offers to buy back bikes that he cannot deliver a rego for, although I don't know anyone who has used this option.
Also expect to wait up to 6 mths for it to be "all clear."
Most likely the bike will first be registered in one of the provinces outside of Bkk, then transferred to Cnx. But if there is anything dodgy about it, then you could have trouble getting it registered in Chiang Mai, as the office here is strict.
If you suspect it aint all lieb-loi, then just keep the bike registered in the original province for 12 mths, renew the rego once, then try to move it "towards" Chiang Mai.